Improve how MergeCommit#merge_commit_message builds the message

Now a merge request with a blank description will no longer produce a merge commit message like this:

Merge branch 'foo' into 'master'

Bring the wonders of foo into the world



See merge request !7283

What an improvement!

Reassigned to @rymai

Added ~480950 ~13921 labels

@rspeicher Please rebase, the build failures are fixed in master. Otherwise it LGTM!

Reassigned to @rspeicher

Added 205 commits:

• 6740c0fd...f4f06d00 - 204 commits from branch master
• b007bd81 - Improve how MergeCommit#merge_commit_message builds the message

Added 205 commits:

• 6740c0fd...f4f06d00 - 204 commits from branch master
• b007bd81 - Improve how MergeCommit#merge_commit_message builds the message

Enabled an automatic merge when the build for b007bd81 succeeds

Status changed to merged

Mentioned in commit 7b42ff63